Hi, new member here. I have been lurking for sometime and found alot of answers to questions but I haven't found anything for this one.

Last year, I bought a 2000 Jimmy 4X4. The engine packed it in shortly thereafter and I decided to try rebuilding it as I've always wanted to try. While everything was apart, I decided to rebuild the entire braking system as most of the stuff was in rough shape. Rebuilt the calipers, new rotors and pads, took apart the master cylinder to clean all the junk out of it. Everything is now back together, started the engine last week and to my surprise, it runs smooth with no codes or other trouble.

My question is about the braking system. I took the entire system apart so everything will need to be bled which I plan to do this weekend. I neglected to bench bleed the MC which I'll do next before gravity bleeding the system. The problem I have is that while everything was together, I pressed on the pedal and noticed that there is an air sound when I press the pedal with the engine off and the master full of fluid. Sounds like its coming from behind the booster. Obviously the pedal goes to the floor as the system has not been bled properly yet. Is this normal or is it possible that the booster is shot as well? It worked when the engine died so it seems weird that it would start randomly with no other changes.

 

I had that air sound over the summer, it was the brake booster. Swapped it for one I got at the scrap yard and been good as new since.

 

That's kinda what I suspected but I don't know much about this braking system yet. Just seems weird that it would start randomly when I took the engine out. And judging by the threads on the difficulty in taking out a booster, I was hoping it would be something else. Do the boosters bring in outside air and the seal is what could possibly be bad?

 

I'm not too sure about what exactly has failed in the booster, all I know is that the one I put in fixed my problem and havent had any issues so far... been several months since I replaced it.

 

Brakes not bled, engine off and no vacuum going to the booster the noise is pretty much normal, before replacing booster bleed brakes and test with engine running.